The liquid crystal display (LCD) technology has made considerable progress in the past years. There are a lot of TV sets, monitors and even public displays based on LCD panels on the market. The market of LCD is expected to keep growing in the near future.
The rapidly growing and changing market sets new tasks for researchers and manufacturers. Growing size of LCD diagonal, which has already exceeded 100 inch size, imposes stronger restrictions onto the quality of optical components. In case of retardation films, very small color shift and ability to provide higher contrast ratio at wide viewing angles are required for high-quality viewing of large displays.
Nowadays there are still some disadvantages of LCD technology which impact the quality of liquid crystal displays and still make feasible the competitive technologies like plasma display panel (PDP) in large-size and cathode-ray tube (CRT) in mid-size displays. One of disadvantages is a decrease of contrast ratio at oblique viewing angles. In conventional LCD the viewing angle performance is strongly dependent upon polarizers' performance. Typical LCD comprises two dichroic polarizers crossed at 90°. However, at oblique angles the angle between projections of their axes deviates from 90°, and polarizers become uncrossed. The light leakage increases with increasing off-axis oblique angle. This results in low contrast ratio at wide viewing angle along the bisector of crossed polarizers. Moreover, generally the light leakage becomes worse because of the liquid crystal cell placed between crossed polarizers.
Off-axis contrast drop issue in LCD can be successfully solved using phase retardation films, which represent optically anisotropic materials. In particular, biaxial retardation films which have three different principal refractive indices can be used for optical compensation of LCD. In contrast with uniaxial retardation films, the biaxial retardation films generally allow providing the best compensation effect using minimal number of compensating sheets.
Phase retardation optical films used for improvement of the LCD contrast at wide viewing angles are known in the art. Most of the conventional phase retardation films are produced by stretching of polymers such as polycarbonate, polyester, polynorbornene etc. Depending on the type of the stress employed on the polymeric film, it is possible to obtain uniaxial or biaxial retardation films of various types. However, the improvement of its performance is difficult due to limitations of the stretching manufacturing process.
Besides the stretching of the amorphous polymeric films, other polymer alignment techniques are known in the art. Thermotropic liquid crystalline polymers (LCP) can provide highly anisotropic films characterized by various types of birefringence. The production of such films comprises coating a polymer melt or solution on a substrate; for the latter case the coating is followed by the solvent evaporation. The additional alignment actions are involved, such as an application of the electric field, using of the alignment layer or coating onto a stretched substrate. The after-treatment of the coating is at a temperature at which the polymer exhibits liquid crystalline phase and for a time sufficient for the polymer molecules to be oriented. Examples of uniaxial and biaxial optical films production can be found in U.S. Pat. No. 5,132,147, and other patent documents and scientific publications.
Optical films can be also produced by coating of lyotropic liquid crystalline (LLC) solutions based on low-molecular compounds capable of forming columnar supramolecules as also known as chromonics. Extensive investigations aimed at developing new methods of fabricating chromonic-based films through variation of the film deposition conditions have been described in U.S. Pat. No. 5,739,296 and other patent documents and scientific publications. Of particular interest is the development of new compositions of lyotropic liquid crystals utilizing modifying, stabilizing, surfactant and/or other additives in the known compositions, which improve the characteristics of the films. The recent research has been directed to the materials used in the manufacturing of anisotropic films, polarizers and retarders for LCD and telecommunications applications, such as (but not limited to) those described in P. Yeh, Optical Waves in Layered Media, New York, John Wiley &Sons (1998), and P. Yeh, and C. Gu, Optics of Liquid Crystal Displays, New York, John Wiley &Sons, (1999).
It has been shown that ultra-thin optically anisotropic birefringent films based on organic dye LLC systems can be produced using the known methods and technologies. In particular, manufacturing of thin crystalline optically anisotropic films based on disulfoacids of dibenzimidazoles of naphthalenetetracarboxylic acid has been described by P. Lazarev and M. Paukshto (in: Proceedings of 7th IDW, (2000), pp. 1159-1160). In the above referenced films the molecules are packed with their minimal polarizability axis parallel to the film coating direction. This structure allows to produce uniaxial −A-plate or biaxial BA-plate retardation film types.
Water soluble rigid-rod polymers are also known to exhibit self-assembled structures in aqueous solutions. Such polymers are used as model objects which are able to reveal some mechanisms taking place in living organisms and implying natural rigid-rod polymers such as deoxyribonucleic acid (DNA), proteins, polysaccharides, having great abilities to form well-ordered structures by spontaneous self-assembly, which is fundamental to invoke their biological functions. As natural rigid-rod polyelectrolytes are difficult to extract without denaturation, synthetic analogues can be studied to investigate some aspects of polyelectrolites aggregation in aqueous solutions. For example, self-assembling properties of water-soluble poly(2,2′-disulfonylbenzidine terephtalamide (PBDT) were investigated by W. Yang et al. (Macromolecules, 41 (5), 1791-1799, 2008). The authors investigated the PBDT sodium salt in different concentration regions. It was shown that at concentration exceeding 3 wt. % the PBDT molecules can form liquid crystalline state. The investigation of electrolyte effect has also shown that adding of the salt (NaCl) enhances the association processes in the PBDT solutions.
Shear-induced mesophase organization of synthetic polyelectrolytes in aqueous solution was described by T. Funaki et al. in Langmuir, 2004, val. 20, 6518-6520. Poly(2,2′-disulfonylbenzidine terephtalamide (PBDT) was prepared by an interfacial polycondensation reaction according to the procedure known in the prior art. Using polarizing microscopy, the authors observed lyotropic nematic phase in aqueous solutions in the concentration range of 2.8-5.0 wt %. Wide angle X-ray diffraction study indicated that in the nematic state the PBDT molecules show an inter-chain spacing, d, of 0.30-0.34 nm, which is constant regardless of the concentration (2.8-5.0 wt %). The d value is smaller than that of the ordinary nematic polymers (0.41-0.45 nm), suggesting that PBDT rods in the nematic state have a strong inter-chain interaction in the nematic state to form the bundle-like structure despite the electrostatic repulsion of sulfonate anions. In the concentration range from 2 to 2.8 wt % a shear-induced birefringent (SIB) mesophase was observed.
A number of rigid rod water-soluble polymers were described by N. Sarkar and D. Kershner in Journal of Applied Polymer Science, Vol. 62, pp. 393-408 (1996). The authors suggest these polymers for different applications such as enhanced oil recovery. For these applications, it is essential to have a water soluble shear stable polymer that can possess high viscosity at very low concentration. It is known that rigid rod polymers can be of high viscosity at low molecular weight compared with the traditionally used flexible chain polymers such a hydrolyzed poly-acrylamides. New sulfonated water soluble aromatic polyamides, polyureas, and polyimides were prepared via interfacial or solution polymerization of sulfonated aromatic diamines with aromatic dianhydrides, diacid chlorides, or phosgene. Some of these polymers had sufficiently high molecular weight (<200,000 according to GPC data), extremely high intrinsic viscosity (˜65 dL/g), and appeared to transform into a helical coil in salt solution. These polymers have been evaluated in applications such as thickening of aqueous solutions, flocculation and dispersion stabilization of particulate materials, and membrane separation utilizing cast films.
Synthesis and properties of solutions of water-soluble polyamides are described by E. J. Vandenberg, W. R. Diveley, et al. in Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 27, pp. 3745-3757 (1989). Poly[N,N′-(sulfo-phenylene)phthalamidles and poly[N,N′-(sulfo-p-phenylene)pyromellitimidel were prepared in water-soluble form and it was found that their solution demonstrate unique properties, in some respects similar to xanthan. The most investigated polymer, poly[N,N′-(sulfo-p-phenylene)terephthalamide] (PPT-S), was produced as the dimethylacetamide (DMAC) salt by the solution polymerization of 2,5-diaminobenzenesulfonic acid with terephthaloyl chloride in DMAC containing LiCl. The isolated polymer requires heating to be dissolved in water; the resulting solutions are viscous solutions or gels at concentrations as low as 0.4%. They are highly birefringent, exhibit circular dichroism properties, and are viscosity-sensitive to the content of salt. Solutions of this polymer mixed with those of guar or hydroxyethyl cellulose give significantly enhanced viscosity. The polymer has a relatively low molecular weight, ca. 5000, as estimated from the viscosity data. Some meta- and para-isomeric analogs of PPT-S were prepared; these polymers have similar properties except they are more soluble in water, and higher concentrations are required to obtain significant viscosity. Poly[N,N′-(sulfo-p-phenylene) pyromellitimide] (PIM-S) was prepared similarly from 2,5-diaminobenzenesulfonic acid and pyromellitic dianhydride. Properties of its aqueous solutions are similar to those of PPT-S. It appears that these relatively low-molecular-weight rigid-chain polymers associate in water to form a network that results in viscous solutions at low concentrations.
Self-assembling properties of sulfonated poly-paraphenylene terephthalamides were considered as a function of number and relative position of sulfonic groups to the main chain by E. Mendes, S. Viale, and S. J. Picken in Proc. Symp. on Functional Polymer Materials, 2004. The authors report, that when the repeated unite contains only one sulfonic group, the structure of the aqueous solutions vary from gel in case of poly(sulfo-paraphenylene terephtalamide) to supramolecular nematic liquid crystal in the case of poly(paraphenylene sulfoterephthalamide). Thus the position of the sulfonic group dramatically affects the structure of the solutions. When two sulfonic groups are present in the repeating unit (in case of poly(sulfo-paraphenylene sulfoterephthalamide)), a molecular polyelectrolyte lyotropic liquid crystal is formed.
The present invention provides solutions to the above referenced disadvantages of the optical films for liquid crystal display or other applications, and discloses a new type of optical film, in particular a biaxial retardation layer.